The invention relates to a novel process, which can be employed on a large industrial scale, for the preparation of a syndiotactic polyolefin.
Syndiotactic polyolefins, in particular syndiotactic polypropylene, are known per se. However, it has not yet been possible to prepare such polymers in an adequate yield under polymerization conditions which are of industrial interest.
Thus, it is known that syndiotactic polypropylene can be prepared by polymerization of propylene at xe2x88x9278xc2x0 C. in the presence of a catalyst system consisting of VCl4, anisole, heptane and diisobutylaluminum chloride (compare B. Lotz et al., Macromolecules 21 (1988), 2375). However, the syndiotactic index (=76.9%) and the yield (=0.16%) are too low.
It is furthermore known that a syndiotactic polypropylene having a mow molecular weight distribution can be obtained in a significantly improved yield with the aid of a catalyst consisting of isopropylene(cyclopentadienyl)(9-fluorenyl)-zirconiun dichloride or isopropylene(cyclopentadienyl)(9-fluorenyl)-hafnium dichloride and a methylaluminoxane at a temperature of 25 to 70xc2x0 C. (compare J. A. Ewen et al., J. Am. Chem. Soc., 110 (1988), 6255). Nevertheless, the molecular weight of the polymer which can be achieved with the zirconium compound is still too low and the yield which can be achieved by means of the hafnium compound is inadequate for an industrial process. Moreover, the syndiotactic characteristics which can be achieved are still in need of improvement.
There was therefore the object of discovering a process which enables syndiotactic polyolefins of high molecular weight, narrow molecular weight distribution and a syndiotactic index of more than 90% to be obtained in a high yield.
The invention thus relates to a process for the preparation of a syndiotactic polyolefin by polymerization or copolymerization of an olefin of the formula RaCHxe2x95x90CHRb, in which Ra and Rb are identical or different and denote a hydrogen atom or an alkyl radical having 1 to 28 carbon atoms, or Ra and Rb, with the atoms joining them, can form a ring, at a temperature of xe2x88x9260 to 200xc2x0 C. under a pressure of 0.5 to 100 bar in solution, suspension or the gas phase in the presence of a catalyst which consists of a metallocene as the transition metal component and an aluminoxane of the formula II 
for the linear type and/or of the formula III 
for the cyclic type, in which, in the formulae II and III, R9 denotes a C1-C6-alkyl group or phenyl or benzyl and n is an integer from 2 to 50, which comprises carrying out the polymerization in the presence of a catalyst, the transition metal component of which is a compound of the formula I 
in which
M1 is titanium, zirconium, vanadium, niobium or tantalum,
R1 and R2 are identical or different and denote a hydrogen atom, a halogen atom, a C1-C10-alkyl group, a C1-C10-alkoxy group, a C6-C10-aryl group, a C6-C10-aryloxy group, a C2-C10-alkenyl group, a C7-C40-arylalkyl group, a C7-C40-alkylaryl group or a C8-C40-arylalkenyl group,
R3 and R4 are different and denote a mono- or polynuclear hydrocarbon radical which can form a sandwich structure with the central atom M1,
R5 is 
xe2x95x90BR6, xe2x95x90AlR6, xe2x80x94Gexe2x80x94, xe2x80x94Snxe2x80x94, xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x95x90SO, xe2x95x90SO2, xe2x95x90NR6, xe2x95x90CO, xe2x95x90PR6 or xe2x95x90P(O)R6, in which R6, R7 and R8 are identical or different and denote a hydrogen atom, a halogen atom, a C1-C10-alkyl group, a C1-C10-fluoroalkyl group, a C6-C10-fluoroaryl group, a C6-C10-aryl group, a C1-C10-alkoxy group, a C2-C10-alkenyl group, a C7-C40-arylalkyl group, a C6-C40-arylalkenyl group or a C7-C40-alkylaryl group, or R6 and R7 or R6 and R8, in each case with the atoms joining them, form a ring, and M2 is silicon, germanium or tin.
The catalyst to be used in the process according to the invention consists of an aluminoxane and a metallocene of the formula I 
In formula I, M1 is a metal from the group comprising titanium, zirconium, vanadium, niobium and tantalum, preferably zirconium.
R1 and R2 are identical or different and denote a hydrogen atom, a C1-C10-, preferably C1-C3-alkyl group, a C1-C10-, preferably C1-C3-alkoxy group, a C6-C10-, preferably C6-C8-aryl group, a C6-C10-, preferably C6-C8-aryloxy group, a C2-C10-, preferably C2-C4-alkenyl group, a C7-C40-, preferably C7-C10-arylalkyl group, a C7-C40-, preferably C7-C12-alkylaryl group, a C8-C40-, preferably C8-C12-arylalkenyl group or a halogen atom, preferably chlorine.
R3 and R4 are different and denote a mono- or polynuclear hydrocarbon radical which can form a sandwich structure with the central atom M1.
R3 and R4 are preferably fluorenyl cyclopentadienyl, it also being possible for the parent structure to carry additional substituents.
R5 is a single- or multi-membered bridge which links the radicals R3 and R4 and denotes 
xe2x95x90BR6, xe2x95x90AlR6, xe2x80x94Gexe2x80x94, xe2x80x94Snxe2x80x94, xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x95x90SO, xe2x95x90SO2, xe2x95x90NR6, xe2x95x90CO, xe2x95x90PR6 or P(O)R6, in which R6, R7 and R8 are identical or different and denote a hydrogen atom, a halogen atom, preferably chlorine, a C1-C10-, preferably C1-C3-alkyl group, in particular a methyl group, a C1-C10-fluoroalkyl group, preferably a CF3 group, a C6-C10-fluoroaryl group, preferably a pentafluorophenyl group, a C6-C10-, preferably C6-C8-aryl group, a C1-C10-, preferably C1-C4-alkoxy group, in particular a methoxy group, a C2-C10-, preferably C2-C4-alkenyl group, a C7-C40-, preferably C7-C10-arylalkyl group, a C8-C40-, preferably C8-C12-arylalkenyl group or a C7-C40-, preferably C7-C12-alkylaryl group, or R6 and R7 or R6 and R8, in each case together with the atoms joining them, form a ring.
M2 is silicon, geranium or tin, preferably silicon or germanium.
R5 is preferably xe2x95x90CR6R7, xe2x95x90SiR6R7, xe2x95x90GeR6R7, xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x95x90SO, xe2x95x90PR6 or xe2x95x90P(O)R6.
The metallocenes described above can be prepared in accordance with the following general reaction scheme: 
Metallocenes which are preferably employed are (arylalkylidene)(9-fluorenyl)(cyclopentadienyl)zirconium dichloride, (diarylmethylene)(9-fluorenyl)(cyclopentadienyl)zirconium dichloride and (dialkylmethylene)(9-fluorenyl)(cyclopentadienyl)zirconium dichloride.
(Methyl(phenyl)methylene)(9-fluorenyl)(cyclopentadienyl-zirconium dichloride, (diphenylmethylene)(9-fluorenyl)(cyclopentadienyl)zirconium dichloride and (dimethylmethylene)(9-fluorenyl)(cyclopentadienyl)zirconium dichloride are particularly preferred here.
The cocatalyst is an aluminoxane of the formula II 
for the linear type and/or of the formula III 
for the cyclic type. In these formulae, the radicals R9 denote a C1-C6-alkyl group, preferably methyl, ethyl, isobutyl, butyl or neopentyl, or phenyl or benzyl. Methyl is particularly preferred. n is an integer from 2 to 50, preferably 5 to 40. However, the exact structure of the aluminoxane is not known.
The aluminoxane can be prepared in various ways.
One possibility is careful addition of water to a dilute solution of an aluminum trialkyl by introducing in each case small portions of the solution of the aluminum trialkyl, preferably aluminum trimethyl, and the water into an initial larger amount of an inert solvent and in between each addition waiting for the evolution of gas to end.
In another process, finely powdered copper sulfate pentahydrate is suspended in toluene in a glass flask and aluminium trialkyl is added under an inert gas at about xe2x88x9220xc2x0 C. in an amount so that about 1 mol of CuSO4.5H2O is available for every 4 Al atoms. After slow hydrolysis, alkane being split off, the reaction mixture is left at room temperature for 24 to 48 hours, during which it must be cooled if necessary, so that the temperature does not rise above 30xc2x0 C. The aluminoxane dissolved in the toluene is then filtered of f from the copper sulfate and the solution is concentrated in vacuo. It is assumed that the low molecular weight aluminoxanes undergo condensation to higher oligomers, aluminum trialkyl being split off, in these preparation processes.
Aluminoxanes are furthermore obtained by a procedure in which aluminum trialkyl, preferably aluminum trimethyl, dissolved in an inert aliphatic or aromatic solvent, preferably heptane or toluene, is reacted with aluminum salts, preferably aluminium sulfate, containing water of crystallization at a temperature of xe2x88x9220 to 100xc2x0 C. In this procedure, the volume ratio between the solvent and the aluminum alkyl used is 1:1 to 50:1xe2x80x94preferably 5:1xe2x80x94and the reaction time, which can be controlled by splitting off the alkane, is 1 to 200 hoursxe2x80x94preferably 10 to 40 hours.
Of the aluminum salts containing water of crystallization, those which have a high content of water of crystallization are used in particular. Aluminum sulfate hydrate is particularly preferred, above all the compounds Al2(SO4)3.16H2O and Al2(SO4)3.18H2O, with the particularly high water of crystallization content of 16 and 18 mol of H2O/mol of Al2(SO4)3 respectively.
Another variant for the preparation of aluminoxanes comprises dissolving aluminum trialkyl, preferably aluminum trimethyl, in heptane or toluene in the suspending agent, preferably in liquid monomer, which has been initially introduced into the polymerization vessel, and then reacting the aluminum compound with water.
In addition to the processes described above for the preparation of aluminoxanes, there are others which can be used. Regardless of the nature of the preparation, all the aluminoxane solutions have the common feature of a varying content of unreacted aluminum trialkyl, which is present in free form or as an adduct. This content has an influence on the catalytic activity which has not yet been precisely clarified, and which varies according to the metallocene compound employed.
It is possible to preactivate the metallocene before use in the polymerization reaction with an aluminoxane of the formula II and/or III. The polymerization activity is in this way increased significantly and the grain morphology improved.
The preactivation of the transition metal compound is carried out in solution. Preferably, in this preactivation, the metallocene is dissolved in a solution of the aluminoxane in an inert hydrocarbon. An aliphatic or aromatic hydrocarbon is suitable as the inert hydrocarbon.
Toluene is preferably used.
The concentration of the aluminoxane in the solution is in the range from about 1% by weight to the saturation limit, preferably 5 to 30% by weight, in each case based on the total solution. The metallocene can be employed in the same concentration, but it is preferably employed in an amount of 10xe2x88x924-1 mol per mol of aluminoxane. The preactivation time is 5 minutes to 60 hours, preferably 5 to 60 minutes. The reaction is carried out at a temperature of xe2x88x9278xc2x0 C. to 100xc2x0 C., preferably 0 to 70xc2x0 C.
A significantly longer preactivation is possible, but this usually neither increases the activity nor reduces the activity, although it may be entirely appropriate for storage purposes.
The polymerization is carried out in a known manner in solution, in suspension or in the gas phase, continuously or discontinuously in one or more stages at a temperature of xe2x88x9260 to 200xc2x0 C., preferably xe2x88x9230 to 100xc2x0 C., in particular 0 to 80xc2x0 C.
The total pressure in the polymerization system is 0.5 to 100 bar. Polymerization in the pressure range of 5 to 60 bar which is of particular industrial interest is preferred. Monomers of boiling point higher than the polymerization temperature are preferably polymerized under normal pressure.
The metallocene compound is used in this reaction in a concentration, based on the transition metal, of 10xe2x88x923 to 10xe2x88x927, preferably 10xe2x88x924 to 10xe2x88x926 mol of transition metal per dm3 of solvent or per dm3 of reactor volume. The aluminoxane is used in a concentration of 10xe2x88x925 to 10xe2x88x921 mol, preferably 10xe2x88x925 to 10xe2x88x922 mol per dm3 of solvent or per dm3 of reactor volume. In principle, however, higher concentrations are also possible.
When the polymerization is carried out as suspension or solution polymerization, an inert solvent which is customary for the Ziegler low pressure process is used. For example, the reaction is carried out in an aliphatic or cycloaliphatic hydrocarbon; examples of these which may be mentioned are butane, pentane, haxane, heptane, isooctane, cyclohexane, and methycyclohexane.
A benzine or hydrogenated diesel oil fraction can furthermore be used. Toluene can also be used. The polymerization is preferably carried out in a liquid monomer.
Olefins of the formula RaCHxe2x95x90CHRb, in which Ra and Rb are identical or different and denote a hydrogen atom or an alkyl radical having 1 to 28 carbon atoms, or wherein Ra and Rb, with the atoms joining them, can form a ring, are polymerized or copolymerized. Examples of such olefins are ethylene, propylene, 1-butene, 1-hexene, 4methyl-1-pentene, 1-octene, norbornene or norbornadiene. Propylene, 1-butene and 4-methyl-1-pentene are preferred.
The polymerization can be of any desired duration, since the catalyst system to be used according to the invention exhibits only a slight time-dependent decrease in polymerization activity.
Polymer powders which consist of compact spherical particles having a very narrow particle size distribution and a high bulk density can be prepared by means of the process according to the invention. The polymer powder is distinguished by very good free-flowing properties.
The polymer has a very high molecular weight, a very narrow molecular weight distribution (polydispersivity) and a very high syndiotactic index. Shaped articles produced from the polymers are distinguished by a high transparency, flexibility, tear strength and an excellent surface gloss.
Polymers of higher molecular weight are formed by using specifically bridged metallocenes according to the invention than by using metallocenes of the prior art. At the same time, the syndiotactic index is improved significantly.
The polymers prepared according to the invention War particularly suitable for the production of films and hollow bodies.
The following examples are intended to illustrate the invention.
In the examples:
VN=viscosity number in cm3/g
Mw=weight-average molecular weight in g/mol
Mn=number-average molecular weight in g/mol
Mw/Mn=polydispersity
The molecular weights were determined by gel permeation chromatography.
SI=syndiotactic index, determined by 13C-NMR spectroscopy
nsyn=average syndiotactic block length (1+IFF/m)
BIH=ball indentation hardness in Nmmxe2x88x922 (measured on flat test speciments 4 mm thick in accordance with DIN 53456/1973)
MFI=melt flow index (230xc2x0 C., 5 kg; DIN 53735)
All the following working operations of the metallocene synthesis are carried out under an inert gas using absolute solvents.